In recent years, decoration film materials such as foil tapes and resin-made tapes are applied to the surfaces of various vehicles including mass transportation systems such as train vehicles, buses, trucks, and passenger aircrafts to differentiate them from others and to display various information such as sense of the season, famous and historical places, landscapes, or advertisements.
Plate-like members can be joined together by placing one on the other and continuously irradiating perpendicularly the surface of one of the plate-like members with a moving laser beam to heat and melt a region extending inside the plate opposite to the plate irradiated with the laser beam. In this case, sufficient joint quality can be ensured by controlling the output power or moving speed of the laser beam such that the bottom of the formed molten pool does not reach the outer surface of the plate opposite to the irradiated plate. However, it is known that a ridge-like weld mark is visually prominent in a fold portion formed by the welding process on the outer surface of the plate-like member opposite to the irradiated plate-like member. In order to address this problem, a technique has already been proposed in which such a weld mark is made less prominent by employing, as the opposite plate-like member, a plate-like member having an outer surface grinding polished in advance substantially parallel to the direction of a weld line to be formed by the laser beam. This technique is particularly suitable for non-painted train vehicle body structures and for bodies of buses and trucks. (See, for example, Patent Document 1.)    [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-329412.